High-order coupled cluster method study of frustrated and unfrustrated quantum magnets in external magnetic fields

摘要: We apply the coupled cluster method (CCM) in order to study ground-state properties of (unfrustrated) square-lattice and (frustrated) triangular-lattice spin-half Heisenberg antiferromagnets presence external magnetic fields. Approximate methods are difficult antiferromagnet because frustration, so, for example, quantum Monte Carlo (QMC) suffers from 'sign problem'. Results this model field rarer than those system. Here we determine solve basic CCM equations by using localized approximation scheme commonly referred as 'LSUBm' carry out high-order calculations intensive computational methods. calculate energy, uniform susceptibility, total (lattice) magnetization local (sublattice) magnetizations a function strength. Our results lattice case compare well QMC approaches all values applied field. find value susceptibility χ = 0.070 antiferromagnet, which is also agreement with other approximate (e.g., 0.0669 obtained via approach). estimate range extent (M/Ms =) plateau 1.37<λ<2.15, good spin-wave theory (1.248<λ<2.145) exact diagonalizations (1.38<λ<2.16). therefore support that indicate begins at higher λ suggested (SWT). The in-plane per site 0.065, below result SWT (evaluated 1/S) χSWT 0.0794. Higher-order thus both LSUBm triangular conclusively.